The subject application is related to subject matter disclosed in the Japanese Patent Application No.Hei11-375842 filed in Dec. 28, 1999 in Japan, to which the subject application claims priority under the Paris Convention and which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to a circuit simulation device for, by using pieces of information related to the element structures and electric characteristics of a plurality of semiconductor elements, predicting the dispersion of circuit characteristics caused by differences of the element structures and the electric characteristics from design values, a circuit simulation method, a circuit simulation program product, and a circuit manufacturing method for, by using pieces of information related to the element structures and electric characteristics of a plurality of semiconductor elements, predicting the dispersion of circuit characteristics caused by differences of the element structures and the electric characteristics from design values, for determining conditions for manufacturing a circuit with reference to the dispersion, and for manufacturing the circuit on the basis of the circuit manufacturing conditions and, more particularly, to a technique for considerably increasing the yield of a circuit manufacturing process.
2. Description of the Related Art
In recent years, with a rapid advance of micropatterning of semiconductor elements, an influence on circuit characteristics caused by differences (=variation in process) of the structures and electric characteristics of semiconductor elements from design values occurring in the process of manufacturing semiconductor elements becomes very conspicuous. For such a background, recently, the following process has been actively performed. That is, the dispersion of circuit characteristics caused by variation in process is predicted by simulation using a computer system, and, with reference to the results, devices and circuits are manufactured.
As a means, which has been proposed up to now, for predicting the dispersion of circuit characteristics caused by variation in process, a means for extracting sets of circuit parameters from the structures and electric characteristics of a plurality of elements which are influenced by the variation in process and for giving the extracted circuit parameter sets to a circuit simulator to obtain a distribution state of the circuit characteristics is generally used.
In the following description, by using a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) as an example, two concrete examples of conventional dispersion predicting methods for circuit characteristics will be introduced.
In the first dispersion predicting method, a plurality of MOSFETs influenced by variation in process in manufacturing are actually measured, or the dispersion of process conditions is given to a process/device simulator, so that pieces of information related to the element structures and electric characteristics of the MOSFETs. Thereafter, by using the pieces of information related to the element structures and the electric characteristics of the MOSFETs are collected, circuit parameters related to the MOSFETs are extracted in a state that gate lengths L (gate widths W) in analytic model equations of the MOSFETs are fixed to a gate length L0 (gate width W0) which is a design value. Subsequently, a plurality of obtained circuit parameters are given to the circuit simulator, and circuit simulation in a state that the gate lengths (gate widths W) in the analytic model equations of the MOSFETs are fixed to the gate length L0 (gate width W0) which is the design value is performed, so that the dispersion of circuit characteristics of the MOSFETs is evaluated.
In this case, as information given to the circuit simulation, in addition to the circuit parameters, a response surface model (model generated on the basis of Response Surface Methodology) representing circuit parameters by using a pricipal component obtained by performing principal component analysis to circuit parameter sets of the plurality of MOSFETs or a corner model may be given. However, the details of these models are omitted.
The xe2x80x9cextraction of circuit parametersxe2x80x9d mentioned here means general processes for determining the values of parameters (to be referred to as circuit parameters hereinafter) in such analytic model equations that electric characteristics obtained by collecting the analytic model equations of the MOSFET incorporated in the circuit simulation.
In addition, the xe2x80x9canalytic model of the MOSFETxe2x80x9d means general equations in which the inter-terminal currents, conductances, capacitances, and terminal charges of the MOSFETs are expressed by the functions of circuit variables such as a terminal bias, gate lengths L, gate widths W, and temperatures T and circuit parameters determined depending on a semiconductor element manufacturing process.
On the other hand, in the second dispersion predicting method, unlike in the first method, when circuit parameters are extracted, gate lengths L (gate widths W) in the analytic model equations of MOSFETs are set to be actual gate lengths Lactual (gate widths Wactual) of the MOSFETs.
Here, when information related to element structures obtained by process/device simulation exists, information related to the actual (=in consideration of variation in process) gate lengths Lactual (gate widths Wactual) can be extracted from the information related to the element structures (more specifically, see PDFAB v2.1 Modeling Reference Manual, PDF Solutions, Inc.). When structure information of real devices exist, the information related to the actual gate lengths Lactual can be extracted by a method in which the sizes of SEM (scanning electron microscope) pictures are measured or other methods.
In this manner, in the conventional circuit manufacturing process, in general, a plurality of circuit parameter sets are extracted from the structures and the electric characteristics of a plurality of elements, and a device manufacturing or circuit manufacturing are performed with reference to a distribution state of circuit characteristics obtained by giving the extracted circuit parameter sets to the circuit simulator. However, the conventional circuit manufacturing process has the following technical problem to be solved.
First, in the conventional circuit manufacturing process, as in the first dispersion predicting method, although the gate lengths and the gate widths of the MOSFETs vary, the gate lengths L (gate widths W) in the analytic model equations are set to be the design values L0 (W0) in extraction of circuit parameters. For this reason, the differences (Lerr=Lactualxe2x88x92L0, Werr=Wactualxe2x88x92W0) between the gate lengths (gate widths W) adversely affect the other circuit parameters. In addition, since the adverse affection changes depending on an extraction strategy of the circuit parameters, parameters which the adverse affection are given cannot be predicted at all. More specifically, in the conventional circuit manufacturing process, since the circuit parameters which are adversely affected by the influence of the errors of the gate lengths and the gate widths cannot be physically and correctly extracted, the circuit characteristics when the gate lengths and the gate widths change cannot be correctly predicted. As a result, the yield of the circuit manufacturing process cannot be improved.
A simple example will be introduced to understand the above problem.
It is assumed that the drain current analytic model equation is given by:
Ids=(W/L)xc2x7U0xc2x7Axc2x7Vds,xe2x80x83xe2x80x83(Equation 1)
and that a drain current Ids is Ids1 when a drain voltage Vds is Vds1.
In this case, when an actual gate length Lactual is given, (equation 1) is given by:
Ids1=(W/Lactual)xc2x7U01xc2x7Axc2x7Vds1.xe2x80x83xe2x80x83(equation 2)
For this reason, by using the value of a parameter value U0, a value U01 which reflects variation in process can be extracted.
However, when the value of U0 is extracted under the condition that L=L0 (L0xe2x89xa0Lactual) is satisfied,
Ids1=(W/L0)xc2x7U01xe2x80x2xc2x7Axc2x7Vds1=(W/L0)xc2x7U01 (L0/Lactual)xc2x7Axc2x7Vds1xe2x80x83xe2x80x83(equation 3)
is satisfied. The influence of Lerr adversely affects the circuit parameter U0,
U01xe2x80x2=U01 (Lactualxe2x88x92Lerr)/Lactualxe2x89xa0U01
is satisfied. A physically correct value cannot be obtained.
Second, in the conventional circuit manufacturing process, unlike in the second dispersion predicting method, information related to variation in the gate length L (gate width W) is not transmitted to the circuit simulation. More specifically, in general, in an analytic model such as BSIM3 (Berkeley Short-channel IGFFT Model) which does not depend on the gate length and the gate width, the electric characteristics of a MOSFET having an arbitrary gate length and an arbitrary gate width are reproduced by one circuit parameter set. The values of the gate length and the gate width do not exist as model parameters, but are newly given in calculation of the electric characteristics by circuit simulation. More specifically, if the model parameters of a model parameter set are extracted as physically correct values, the model parameter set do not include information related to variation in the gate length and the gate width. As a result, the information, which is included in the original data, related to the variation in the gate length and the gate width are lost, the dispersion of the circuit characteristics is underestimated by the lost information. As a result, the yield of the circuit manufacturing process cannot be improved.
The present invention has been made in consideration of the above technical problems, and has as its object to provide a circuit simulation device for considerably improving the yield of a circuit manufacturing process.
It is another object of the present invention to provide a circuit simulation method for considerably improving the yield of a circuit manufacturing process.
In addition, it is still another object of the present invention to provide a computer readable recording medium in which a circuit simulation program for considerably improving the yield of a circuit manufacturing process is stored.
Furthermore, it is still another object of the present invention to provide a circuit manufacturing method for considerably improving the yield of a circuit manufacturing process.
For the technical problems described above, the present inventors correctly reflect the magnitude of the dispersions of gate lengths and gate widths on a specific circuit parameter to extract another circuit parameter, so that the influence of the dispersions of the gate lengths and the gate widths is prevented from adversely affecting circuit parameters except for the specific circuit parameter. For this reason, the circuit parameters can be correctly extracted, and circuit characteristics can be accurately predicted. As a result, the present inventors thought that manufacturing conditions for a circuit having desired circuit characteristics could be correctly determined to make it possible to perform a semiconductor manufacturing process at a high yield.
According to the present invention, since the magnitudes of the dispersions of gate lengths and gate widths are included in a specific circuit parameter, even though a distribution is not given to a description portion of the gate lengths and the gate width in execution of circuit simulation, the influence of the dispersions of the gate lengths and the gate widths can be reflected on a simulation result. Circuit characteristics can be evaluated at a high accuracy in consideration of the influence of variation in process.
According to the present invention, since actual gate lengths and actual gate widths are used in extraction of circuit parameters, the circuit parameters can be extracted at a high accuracy.
Furthermore, according to the present invention, when circuit characteristics are to be analyzed, dispersion can be given to gate lengths and gate widths. For this reason, the dispersion of circuit characteristics can be correctly evaluated, and the magnitude of the dispersion can be correctly predicted.
Other and further objects and features of the present invention will become obvious upon understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the invention in practice.